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Old 20-09-2021, 09:43 AM
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mura_gadi (Steve)
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Kelley-Alder Mirror Flex System

Hello,

I came across this article from sky and telescope and thought it held some great promise. In principle it pulls the mirror from spherical to paraboloid, eliminating parabolisation from the mirror making progress. The thickness of the glass and the focal lengths works well for the telescope projects I have planned.

Has anyone seen or used a mirror/mirror cell based on this design?


Thanks
Steve


https://skyandtelescope.org/wp-conte...MirrorFlex.pdf
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Old 20-09-2021, 11:32 AM
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multiweb (Marc)
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Couple of things come to mind. Changes in temperature, changes in focal length and focus, aberrations due to uneven glass stress.
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Old 20-09-2021, 12:36 PM
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Hello,

I'd imagine its not a quick fix, the need to level the back and smooth it. Changes in temp are the same, you have to recalibrate if you have changed a few degrees since your last one anyway. I think the focal length change was covered when they star tested and adjust the tension bolt further, again the F/L dictates the tolerances.

Aberrations/astigmatism plague of any cell design - The contact material would need to be of even compression and adhesion etc. The weight of the mirror should be better distributed over the donut. Might also be a little easier than the 9/18 pointing float systems.

But to get a 16" F5 spherical to paraboloid without parabolising must be worth 50+ hours in my case. If (big if) I can acquire the skills along the way to manage such a blank to a useable wave front.

Certainly going to try with my current 8", I was split between cell designs to hold the mirror for a standard newt on tripod setup as well (retaining clips etc ).

With the Kelley-Alder mirror cell design, if you don't introduce astigmatism, it will hold the mirror for either setup and not use clips, retainers, straps etc which can pinch or distort the mirror.


Steve
Ps. Well I'll give it a try unless someone points out it fatal flaws... I am assuming that there must be some as it should have taken off as an option for some diameter/focal length/glass thickness combo's...

Last edited by mura_gadi; 20-09-2021 at 12:50 PM.
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Old 21-09-2021, 06:13 PM
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billdan (Bill)
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I have read about mirror flexing and I believe it only works with slow mirrors i.e F7 or slower.
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Old 22-09-2021, 06:07 PM
SkyWatch (Dean)
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I tried this with my 10" f5.6, 1" thick plate glass mirror that needed a slight tweak. I glued the puller to the centre of the mirror (based on an earlier article: although he does mention this again at the end of the referred article). It worked well and gave me perfect star images. It is probably worth a try with the 8" as a "proof of concept".

One issue: the first glue that I tried was too strong (an industrial strength epoxy), and it shrank slightly as it hardened, which did strange things to the star tests... Once I cleaned this off I used a lower strength epoxy glue and it was fine.

Eventually though I found it was a bit of a pain because I needed to tweak the puller when there were major temperature changes, and I ended up re-figuring the mirror.

- Dean
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Old 22-09-2021, 06:24 PM
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mura_gadi (Steve)
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Hello,

Thanks for the reply, was the need to tweak the puller more painful due to getting the puller adjusted correctly? Or the puller adjustment bolt needed adjusting frequently due to temperature drop? What sort of temperature variation/range did you encounter?

Did you release the tension after every session? Or see any issues if you had left the mirror tensioned for a period of time?



Thanks again
Steve
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Old 23-09-2021, 09:36 AM
SkyWatch (Dean)
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Quote:
Originally Posted by mura_gadi View Post
Hello,

Thanks for the reply, was the need to tweak the puller more painful due to getting the puller adjusted correctly? Or the puller adjustment bolt needed adjusting frequently due to temperature drop? What sort of temperature variation/range did you encounter?

Did you release the tension after every session? Or see any issues if you had left the mirror tensioned for a period of time?



Thanks again
Steve
Hi Steve,

This was a long time ago (and not long before the scope was stolen anyway!!!). From memory it was just one of those things that needed doing, along with checking collimation, after I had travelled somewhere and assembled the scope. No idea whether it was temperature changes affecting the bolt or the mirror, or both. I found it surprising how easily the view was affected by a tiny turn of the screw.

- and yes, I released the tension a bit after sessions.

If you have a full thickness pyrex mirror you might find things behave a bit differently.

All the best,

Dean
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